Intake valves control entry of an air/fuel mixture into cylinders of an internal combustion engine. Exhaust valves control gases exiting the cylinders of an internal combustion engine. Camshaft lobes (or “cam lobes”) on a camshaft push against the valves to open the valves as the camshaft rotates. Springs on the valves return the valves to a closed position. The timing, duration and degree of the opening, or “valve lift,” of the valves can impact performance.
As the camshaft rotates, the cam lobes open and close the intake and exhaust valves in time with the motion of the piston. There is a direct relationship between the shape of the cam lobes and the way that the engine performs at different speeds and loads. When running at low speeds, the cam lobes should ideally be shaped to open the intake valve as the piston starts moving downward in the intake stroke. Generally, the intake valve should close as the piston reaches the bottom of its stroke and then the exhaust valve opens. The exhaust valve closes as the piston completes the exhaust stroke at the top of its stroke.
At higher engine speeds, however, this configuration for the cam lobes does not work as well. If, for example, the engine is running at 4,000 RPM, the valves are opening and closing 33 times every second. At this speed, the piston is moving very quickly. The air/fuel mixture rushing into the cylinder is also moving very quickly. When the intake valve opens and the piston starts the intake stroke, the air/fuel mixture in the intake runner starts to accelerate and move into the cylinder. By the time that the piston reaches the bottom of its intake stroke, the air/fuel mixture is moving at a high speed. If the intake valve is shut quickly, all of the air/fuel flow stops and does not enter the cylinder. By leaving the intake valve open longer, the momentum of the fast-moving air/fuel mixture continues flowing into the cylinder as the piston starts its compression stroke. The faster the engine turns, the faster the air/fuel mixture moves and the longer the intake valve should stay open. The valve should also be opened to a greater lift value at higher speeds and higher loads. This parameter, called “valve lift,” is governed by the cam lobe profile. A fixed cam lobe profile which always lifts the valve the same amount does not work well at all engine speeds and loads. Fixed cam lobe profiles tend to compromise engine performance at both idle and at high loads.
Variable valve actuation (VVA) technology improves fuel economy, engine efficiency, and/or performance by modifying the valve event lift, timing, and duration as a function of engine operating conditions. Two-step VVA systems enable two discrete valve events on the intake and/or exhaust valves. The engine control module (ECM) selects the optimal valve event profile that is best utilized for each engine operating condition.
An issue in the development and application of the two-step VVA system is the response time variability of a Control Valve (CV) and VVA hydraulic control system. A limited amount of time is available for switching two-step Switching Roller Finger Followers (SRFF) between engaging in one valve event and the corresponding part of the next valve event of another engine cylinder controlled by the same CV. If the CV causes a fluid pressure change in the lifter fluid gallery to occur too soon relative to the critical part of a valve lift curve, the SRFF arm lock pin may only partially engage and then disengage after the valve has started lifting. This unscheduled disengagement is called a “Critical Shift” and may cause the engine valve to drop uncontrollably from the high-lift valve event to the low-lift valve event, or on to the valve seat. After a number of such events, the SRFF arm or the valve may show signs of accelerated wear or damage.
Several factors can affect hydraulic system variation including but not limited to engine oil aeration, duration of engine operation, wear upon the components of the engine, degradation of fluid quality over time, engine temperature, and/or fluid viscosity. These factors increase hydraulic system variations among engines and contribute to the accelerated wear and damage to the engine components.